The electrophoretic mobility of restriction fragments of lambda DNA in agarose gels declines if the field is intermittent rather than continuous, with a greater effect on the longer fragments. The changes are compatible with the assumption of two exponential relaxation processes for fielddependent configurational changes, one when the field is turned on and another when it terminates. The length dependence at the extrapolated limit of mobility for short pulses with long intervals corresponds closely to the simple inverse proportionality to length expected from theoretical considerations when the molecular configuration is not affected by the electric field. Simple intermittent fields would allow separation of longer molecules than can ordinarily be resolved. The relaxation times for both the change in conformation imposed by the field and the return to field-free conformation vary BS approximately the second power of the length of the molecule, independent of the salt concentration or field strength and varying only slightly with gel density. These relations are not in good agreement with properties expected from reptation theory, and they suggest that a different mechanism must be invoked for the electrophoretic migration of long DNA molecules at ordinary values of field strength.